The present invention relates generally to power conversion systems and deals more particularly with a DC-to-AC high power inverter having a staggered resonant recovery system to recover switching energy losses for return to the DC voltage supply.
It is usual for high power inverters to operate at frequencies much higher than the required frequency of the output voltage. In motor drives, for example, the output voltage of the inverter can be varied in a series of small steps to create the desired voltage waveshape. Typically, the output voltage waveshape is sinusoidal with the inherent motor inductance being used to filter out the high frequency components of the output voltage. In utility voltage supply inverters, the high frequency filtering of the output voltage is generally provided by separate inductors which at high frequencies are inherently smaller in size than a single inductor.
Multiple power semiconductor switching devices are used to achieve higher power output from the inverter since each of the multiple switching devices can have a lower rating because all handle a portion of the current supplied to the load rather than all of the current. One known technique employing multiple power semiconductor switching devices in a high power inverter utilizes a staggered pole(s) in unipolar and bipolar configurations wherein each pole or pair of poles has a separate load inductor, and the switching or current handling is distributed substantially evenly among the poles throughout the high frequency period.
One limitation of the foregoing staggered pole approach is due to the switching losses that occur in the semiconductor switching devices. The semiconductors are subject to switching losses which are proportional to the switching frequency and such losses are often greater than conduction losses in high frequency applications. Techniques to reduce the switching losses are known as soft switching and use of such techniques lower the operating temperature output. Such soft switching techniques for a given inverter module include auxiliary resonant commutated pole (ARCP) and a diode/capacitor snubber circuit across each semiconductor switching device.
The ARCP technique enables zero turn-off loss of the main switching device by the addition of an auxiliary triggered resonant commutation snubber circuit to commutate the load current from a main diode to another active semiconductor device. The ARCP technique requires another semiconductor switching device with substantially the same peak current rating as the main semiconductor switching device and does not lend itself to assisting multiple commutations because the peak current of the resonant pole must be distributed among the main semiconductor switching devices.
The diode/capacitor snubber circuit technique maintains a low voltage across the semiconductor switching device while the device turns-off and the current decays. If the capacitor is large enough to maintain less than the supply voltage for the entire turn-off period, the capacitor must continue to carry load current until the capacitor voltage reaches the magnitude of the supply voltage. This technique results in storing substantially more energy in the capacitor than the switching losses saved. Furthermore, the energy stored in the capacitor must be dissipated or converted by means of another switching converter to the output voltage or to the DC voltage supply bus.
The above disadvantages are solved with a staggered resonant recovery circuit of the present invention which provides a means for recovering most of the energy stored in the snubber capacitor by reversing the snubber capacitor voltage with respect to the midpoint of the DC voltage supply and returning the stored energy to the DC voltage supply.
A further advantage of the staggered resonant recovery circuit of the present invention is one inductor may operate with multiple capacitor/diode snubber circuits to provide a soft reverse recovery for the positive and negative switching devices in one phase of the power inverter returning most of the switching energy losses to the output voltage or to the DC supply bus thus significantly improving the form factor of the inductor current.